[docs] Document Django Channels WebSocket endpoints

[nemesifier]

Document Django Channels WebSocket API

Context: This repository implements Django Channels WebSocket endpoints for topology updates (admin and non-admin views). There is no dedicated documentation describing these endpoints or their message types in the docs/ folder.

Example endpoint

• Topology updates: ws://<host>/ws/network-topology/topology/<topology_id>/

Message Types

• E.g., send_topology_update (payloads with topology JSON data)

Suggestion:
Please create documentation in docs/user/websocket-api.rst (or appropriate location) covering:

• WebSocket endpoint URLs, message types, and sample payloads
• Authentication/authorization requirements
• Purpose: Help integration developers and clarify API for maintainers

This is modeled after the suggestion from openwisp-firmware-upgrader.